Extracellular pH, Ca(2+) influx, and response of vascular smooth muscle cells to 5-hydroxytryptamine.

BACKGROUND AND PURPOSE

Cerebral vascular smooth muscle cells (VSMCs) contract on extracellular pH (pH(o)) increases and relax on pH(o) decreases. These changes in tone are believed to result from changes in Ca(2+), although the responsible mechanisms are not fully understood. VSMCs also contract in response to 5-hydroxytryptamine (5-HT), which increases Ca(2+) via both Ca(2+) release and influx. We hypothesized that examining effects of pH(o) decreases on 5-HT-induced Ca(2+) changes would allow us to identify mechanisms whereby pH(o) influences tone. Accordingly, we compared Ca(2+) increases in cerebral VSMCs, evoked by 5-HT, with increases evoked by increased pH(o) and examined 5-HT-dependent Ca(2+) increases at normal and decreased pH(o).

METHODS

We monitored Ca(2+), using the Ca(2+)-sensitive dye fura 2, in cultured rat cerebral VSMCs obtained by enzymatic digestion of middle cerebral arteries and their branches (passages 1 to 3) grown on glass coverslips and superfused with physiological saline.

RESULTS

Increasing pH(o) from 7.3 to 7.8 increased Ca(2+), and these increases were prevented in Ca(2+)-free solutions. Decreasing pH(o) from 7.3 to 6.9 did not alter Ca(2+) unless Ca(2+) was first raised by treatment with 5-HT (10 micromol/L). 5-HT resulted in biphasic Ca(2+) increases characterized by transient peaks blocked by the Ca(2+)-ATPase inhibitor thapsigargin (10 nmol/L) and prolonged plateaus blocked by the Ca(2+) channel blocker Ni(2+) (1 mmol/L). Acidification did not alter the transient peaks but significantly reduced 5-HT-induced Ca(2+) influx.

CONCLUSIONS

We conclude that increasing pH(o) induces Ca(2+) influx in rat cerebral VSMCs and decreasing pH(o) inhibits 5-HT-stimulated Ca(2+) entry but not intracellular Ca(2+) release.